In inductive flow measurements with constant (d.c.) field of the direct current voltage induced in a flowing fluid, the amplitude of which induced voltage is of the order of magnitude of a few microvolts, in order to separate this induced voltage from the electrochemical potential, which can reach up to 10.sup.5 -- fold values of the actual current being measured, it is known from DT-AS 2 052 175 to compensate the electrochemical potential with the use of a periodically switched constant (d.c.) field, the perturbing potential is stored with the magnetic field switched off and then, from the total signal produced with the magnetic field switched on, the required voltage and the perturbing potential are separated. Alternatively the magnetic field is alternately reversed in direction, so that the required voltage changes its polarity, while the perturbing potential maintains its polarity. Compensation is achieved in that the total signal of the required voltage and perturbing potential and the difference signal of the perturbing potential minus the required voltage is stored and from the difference of the two stored signals the required signal is determined, freed from perturbing potentials.
These methods require however a high expenditure on circuitry, in order to detect with the necessary exactness the required signal, which is smaller than the perturbing signal by a factor of about 10.sup.5, in the differencing process. Furthermore, for the accomplishing of this method, two stores and a difference amplifier are required. In this connection it cannot be avoided that the intrinsic errors in the stores and difference amplifiers may possibly add, so that from this cause also a mistake may arise in the measurement.
An additional method is disclosed in the above cited prior art document, namely, periodically short-circuiting the measurement electrodes and the input direct current voltage amplifier, and measuring and storing the required voltage for a short time directly after the removal of this short-circuiting, before it is falsified by the slow build-up of the electrochemical potential. A range of perturbing side-effects arise with this prior art method because of the effect which is thereby exerted on the actual measurement standard, which makes it very difficult to measure the required voltage with the necessary accuracy.